Fogging and icing are persistent problems for passenger vehicles, aircraft, watercraft, trains, residential and commercial buildings, and all other devices that incorporate glass surfaces that are exposed to significant ambient air temperature gradients. The condition commonly known as fogging is caused by condensed water vapor collecting on a glass surface due to the difference in temperature between the glass and the adjacent air. This is a common problem for many types of vehicles (including cars, trucks, aircraft, watercraft, and trains) because the inside temperature of the vehicle often must differ significantly from the outside temperature to maintain passenger comfort. For example, if the interior temperature of a car is warmer than the outside temperature (as when a car is left outside in cold weather), the inside surface of the car windshield will be cooler than the air inside the vehicle. Air in contact with the inside surface of the windshield will thus cool down through contact with the glass. The cooling of this air reduces its ability to retain moisture, and thus the moisture that is released condenses on the inside surface of the windshield. Heating the inside surface of the windshield (as through a traditional car defroster system) will vaporize this condensed water, and will also prevent condensation from again appearing on the surface while the defroster is activated. This same problem is encountered with windows in residential and commercial buildings because of the difference in outside and inside temperature.
Icing, like fogging, can also be effectively removed or prevented through the heating of a windshield or other glass surface. Typically, icing on a vehicle's windshield occurs when the vehicle is not in motion and thus must be removed before the vehicle may be safely operated. The vehicle operator must then either wait for the vehicle defroster to melt the ice (which typically requires several minutes with a traditional defroster system), or must scrape the ice manually and suffer the outside cold temperatures during the process.
A particularly dangerous situation may occur when water at or near the freezing point strikes the relatively cool windshield of a vehicle while it is in motion, such as when cold water is splashed up onto a car windshield by a passing tractor trailer. Under certain conditions, water splashed up in this manner can "flash freeze" across a windshield virtually instantaneously, blocking a driver's vision entirely. Rapid, direct heating of the ice is necessary to quickly restore the operator's vision and thereby prevent a potentially life-threatening accident.
Traditional vehicle defroster mechanisms derive heat from the vehicle's engine coolant system. Liquid coolant passing through the engine block absorbs heat, thereby removing heat from the engine. The coolant then passes through a radiator or other type of heat exchanger where the coolant's heat is expelled and the coolant is ready to be passed through the engine block again in a closed loop. To heat the air for traditional vehicle defroster systems, air is blown across a heat exchange system through which the heated engine coolant passes, thereby transferring the heat energy from the coolant to the air. This heated air is then pushed by the blower through ductwork in the dashboard of the vehicle and onto the vehicle's windshield through large vents at the base of the windshield.
Several disadvantages result from this traditional approach. First, the air passing through the blower cannot be heated until the engine coolant is warmed due to engine fuel combustion. As a result, it may be several minutes after the engine begins running before the air being blown against the vehicle windshield is sufficiently heated to have any effect on fogging or icing. Second, the heated air is usually directed against the windshield in only one or a few locations, typically near the bottom of the front windshield. Consequently, only a small portion of the windshield is initially exposed to the heated air, and the remaining portions of the windshield are only gradually heated as the heated air slowly fills the passenger compartment through one or a few restricted openings. Third, there is no means to directly heat the exterior of the windshield while the interior is being heated; instead, the windshield exterior is only heated through the conduction of heat through the glass to the exterior side. Glass is a poor conductor of heat, resulting in still more delay and wasted energy while the glass is slowly heated through from the interior side before any defogging or deicing effect is realized on the exterior of the windshield. Fourth, traditional heater systems require a substantial amount of ductwork and a number of vents stretching through the passenger compartment to provide heat to occupants. Most vehicle heater systems provide uneven heat to the passenger compartment, with the front portion of a vehicle heating first and those passengers in the rear remaining uncomfortable until convection finally carries heat to the rear of the passenger compartment. Somewhat more even heating may be achieved through the use of additional ductwork and vents directed into the rear portion of the passenger compartment, but initial heating is still concentrated only in proximity to those handful of vents. Moreover, the additional ductwork required to deliver heat to rear passengers is bulky and thus reduces the passenger and cargo space within the vehicle.
Several attempts to improve the performance of traditional heater and defroster systems have been made. U.S. Pat. No. 2,304,691, issued to F. C. Hund, discloses a windshield defroster for an aircraft that distributes heated air along all edges of the windshield. Hund '691 teaches a double-pane windshield with the heated air being introduced between the panes of glass.
U.S. Pat. No. 2,672,735, issued to J. Fusselman, discloses a device to prevent condensation on refrigerated display case windows. Fusselman '735 teaches that a series of air outlets appear above and below the display case window, and distribute hot air across the exterior surface of the window. The display case's refrigeration unit provides the necessary heat for the unit.
U.S. Pat. No. 2,718,037, issued to A. R. Lintern, discloses a finned tube heater and defroster unit for a vehicle. Lintern '037 teaches a long tube extending beneath the length of a windshield, through which hot liquid from the engine coolant system is passed. The tube comprises a plurality of fins extending therefrom, over which air is blown from one end of the tube. The air is heated as it passes over the finned tube, and the fins direct the air upward against the inside surface of the windshield.
U.S. Pat. No. 5,173,586, issued to Peter N. Gold, discloses an electric heating attachment to free frozen windshield wipers resting at the bottom portion of the exterior of a vehicle windshield. The Gold '586 device extends along the bottom of a vehicle windshield and contacts the blade of windshield wipers resting against it.
U.S. Pat. No. 5,206,476, issued to John B. Fresch et al., teaches a supplementary vehicle heater that is placed in-line with a traditional vehicle heater system. The Fresch et al. '476 device comprises a polymer block that heats in reaction to the passage of electrical current therethrough. The device also has a number of holes through which air may pass. As air is pushed from the vehicle heater blower through the passages in the heated polymer block, the air is heated. This device is intended to heat air prior to the heating of engine coolant during the first few minutes after the vehicle is started.
None of these devices provide an ideal solution to the problem of heating, defrosting, and deicing within a passenger vehicle. In particular, none of these devices are useful to heat the interior and exterior of a vehicle windshield or other glass surface simultaneously, and therefore cannot quickly and efficiently remove fogging or icing no matter which surface of the windshield the fogging and icing appears on. Moreover, none of these devices effectively solve the special and extremely dangerous problem of "flash icing" which, as described above, may completely blind the operator of a vehicle while that vehicle is in motion. The slow conduction of heat from the inside of a windshield to the outside cannot remove ice quickly enough to safely clear the vision of the vehicle's operator. Finally, none of these devices are useful to heat the interior passenger compartment of a vehicle evenly and efficiently without the necessity of the multitude of vents and ductwork found in cars utilizing traditional heater systems.